Enlightenment is man's emergence from his self-incurred immaturity. Immaturity is the inability to use one's own understanding without the guidance of another. This immaturity is self-incurred if its cause is not lack of understanding, but lack of resolution and courage to use it without the guidance of another. The motto of enlightenment is therefore: Sapere aude! Have courage to use your own understanding!

Pauca sed matura.
(Few, but ripe.)
His motto. He would limit his publications to work he regarded as complete and perfect.

Thou, nature, art my goddess; to thy laws my services are bound...
His second motto (from King Lear by Shakespeare).

According to my views, aiming at quantitative investigations, that is at establishing relations between measurements of phenomena, should take first place in the experimental practice of physics. By measurement to knowledge [door meten tot weten] I should like to write as a motto above the entrance to every physics laboratory.

Genius is two percent inspiration, ninety-eight percent perspiration.

Good teachers deserve apples; great teachers deserve chocolate.
A favorite quotation, written in calligraphy on his office door.

The motto of all the mongoose family is, 'Run and find out'.